1. Field of the Invention
The present invention relates to angular velocity sensors, and more particularly relates to an angular velocity sensor for detecting angular velocity based on the oscillation output from a vibrating gyroscope which uses a bimorph vibrator which is used in navigation systems and for correcting camera movement caused by hand shake.
2. Description of the Related Art
FIG. 4 is a perspective view of an example of a bimorph vibrator used in an angular velocity sensor. FIG. 5 is a waveform diagram of an oscillatory wave output from the bimorph vibrator shown in FIG. 4.
Referring to FIG. 4, a bimorph vibrator 1 is formed of two piezoelectric elements pasted together so that their polarization directions are opposite to each other and so that their cross sections are rectangular. The vibrator 1 vibrates in a longitudinal vibration mode so that it vibrates, in the X-axis direction, perpendicularly to the faces. When the vibrator 1 is rotated in the Z-axis direction at a particular angular velocity (xcfx89), vibrations result in a transverse vibration mode in the Y-axis direction which is perpendicular to the drive surfaces, due to Coriolis force.
The amplitude of the vibrations is proportional to the angular velocity. By utilizing this characteristic, the angular velocity value can be determined. The vibrator 1 is provided with a left electrode, a right electrode, and an overall electrode (none of which are shown). Referring to FIG. 5, an L (left) signal (a) and an R (right) signal (b) are output from the left electrode and the right electrode, respectively. The L signal and the R signal have slightly different amplitudes and phases. The difference between the L signal and the R signal is the Lxe2x88x92R signal (c), and the sum of the L signal and the R signal is the L+R signal (d).
Concerning the Lxe2x88x92R signal, the larger the phase lag between the L signal and the R signal, the further the zero crossing point is moved. The Lxe2x88x92R signal is also referred to as a null difference voltage. The Coriolis force is added to the Lxe2x88x92R signal, and the Lxe2x88x92R signal is output as the sum of the difference and the Coriolis force. It is impossible to isolate the Coriolis force shown in FIG. 5, because the Coriolis force is not output as an actual signal. Instead, the Coriolis force is output virtually. In the following description, it is assumed that the Lxe2x88x92R signal is the sum of the difference and the Coriolis force. The Coriolis force (e) is in phase with the L+R signal (d). The Coriolis force (e) reaches its maximum value and minimum value in the vicinity of the maximum point and the minimum point of the L+R signal (d), respectively. When the vibrator 1 is swayed from side to side, as shown in FIG. 5, the phase of the Coriolis force (e) varies. In contrast, the phase of the L signal (a) and the phase of the R signal (B) do not vary.
The above-described vibrator 1 is required to separately adjust the balance, null voltage (which is also referred to as an off-setting voltage or a neutral voltage), and sensitivity.
FIG. 6 is a block diagram of an angular velocity detecting circuit for obtaining the output of the vibrator 1 shown in FIG. 4. Referring to FIG. 6, the differential output of the vibrator 1 is amplified by a differential amplifier circuit 201. The amplitude waveform is detected by a synchronous detector circuit 202. The detected waveform is smoothed by a smoothing circuit 203 to output a DC voltage. The DC voltage is DC-amplified by a DC amplifier 204. When the DC amplifier 204 amplifies the signal, the null voltage is also amplified. Accordingly, a DC cutting circuit 205, which is formed of a filter, cuts the DC component of the output from the DC amplifier. An amplifier circuit 206 amplifies the output of the DC cutting circuit 205 and outputs an analog signal. The analog signal is converted into a digital signal by an analog-to-digital (A/D) converter 207. An angular velocity detection signal is supplied to a microprocessor 208 to suppress camera vibration movement or to perform navigation control.
In the angular velocity detecting circuit shown in FIG. 6, since the source sensitivity of the vibrator 1 is low, it is required that the DC amplifier 204 amplify the signal for a gain of 20 dB. When the reference level is shifted due to temperature characteristics of the null voltage, and when DC amplification is performed, the null voltage occasionally exceeds the supply voltage. Therefore, it is necessary to limit the degree of DC amplification. To this end, the DC cutting circuit 205 is provided, and amplification is again performed by the amplifier circuit 206. As a result, there is an increase in the number of circuit components.
Recently, significant improvements have been made in microprocessors and digital processors (digital signal processors (DSPs)). There has also been a reduction in cost. When an analog signal is converted into a digital signal at the earliest stage as possible, the total cost of a system is reduced.
In particular, for devices such as pointing devices which detect angular velocity at low cost and which require two axes, the cost of the devices increases as it becomes necessary to double the number of peripheral circuits such as the synchronous detector circuit 202, the smoothing circuit 203, and the DC amplifier 204.
Accordingly, it is an object of the present invention to provide an angular velocity sensor for directly obtaining an angular velocity signal using a relatively simple circuit configuration, and without using redundant circuits.
According to an aspect of the present invention, an angular velocity sensor is provided for driving a vibrator in the X-axis direction and for detecting angular velocity based on vibrations caused by a Coriolis force generated in the Y-axis direction when the vibrator rotates about the Z-axis. The angular velocity sensor includes a driver for generating a reference signal based on left and right signals or a differential signal output from the vibrator and driving the vibrator. A signal extracting unit extracts the left and right signals or the differential signal output from the vibrator, in which the signals include the Coriolis force. A converter converts the left and right signals or the differential signal output from the vibrator, the signals including the Coriolis force, into at least one digital signal. An arithmetic operation unit generates xcfx80/2-phase-shifted left and right signals or a xcfx80/2-phase-shifted differential signal based on the digital left and right signals or the digital differential signal, respectively, the signals being digitized by the converter, computes the sum of squares of the xcfx80/2-phase-shifted left and right signals and the original left and right signals or computes the sum of squares of the xcfx80/2-phase-shifted differential signal and the original differential signal, and computes and outputs a magnitude signal in proportion to the Coriolis force.
The arithmetic operation unit may include a phase circuit, such as a Hilbert transformer for shifting the phase of the differential signal by xcfx80/2. A first multiplier circuit may square the original differential signal. A second multiplier circuit may square the xcfx80/2-phase-shifted differential signal. An adder circuit may add the output of the first multiplier circuit and the output of the second multiplier circuit. A square root circuit may compute the square root of the output of the adder circuit.
The arithmetic operation unit may include phase circuits, such as Hilbert transformers, for shifting the phase of the respective left and right signals by xcfx80/2. First multiplier circuits may square the respective original left and right signals. Second multiplier circuits may square the respective xcfx80/2-phase-shifted left and right signals. A first adder circuit may add the squared left signal obtained by the corresponding first multiplier circuit and the squared xcfx80/2-phase-shifted left signal obtained by the corresponding second multiplier circuit. A second adder circuit may add the squared right signal obtained by the corresponding first multiplier circuit and the squared xcfx80/2-phase-shifted right signal obtained by the corresponding second multiplier circuit. A first square root circuit may compute the square root of the output of the first adder circuit. A second square root circuit may compute the square root of the output of the second adder circuit. A subtracter circuit may compute the difference between the outputs of the first and the second square root circuits, divide the difference in two, and output the halved difference.
According to the present invention, it is possible to easily extract a magnitude signal which is proportional to Coriolis force by converting L and R signals or a differential signal output from a vibrating gyroscope into a digital signal(s) and computing the sum of squares of the original signal(s) and the xcfx80/2-phase-shifted signal(s).
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.